Two-component coating compositions comprising a polyisocyanate component binder in combination with a reactive component that is reactive toward isocyanate groups, in particular a polyhydroxyl component, have long been known. They are suitable for producing high-grade coatings, which can be made hard, elastic, abrasion-resistant and solvent-resistant.
In view of the fact that primary amines undergo a spontaneous and rapidly uncontrollable reaction with polyisocyanates they are unsuitable as reaction partners for polyisocyanates in two-component coatings.
EP-A 0 403 921 describes low-viscosity polyaspartic esters which contain secondary amino groups and whose reactivity towards NCO groups is therefore moderate as compared with primary amines. Polyaspartic esters of this kind are particularly suitable for preparing low-solvent or solvent-free coating compositions which exhibit rapid curing.
Polyaspartic esters of this kind are, however, not water-soluble or water-dispersible, and so are not suitable for use in aqueous binders.
In view of the evermore stringent legislation governing permitted levels of volatile organic components in, for example, coating compositions, there is increasing demand for aqueous systems. Aqueous two-component coating compositions have been known for years and are described for example in EP-A 0 358 979.
EP-B 0 818 492 describes aqueous secondary polyamines which are obtained by Michael Addition of polyamines with oligoesters containing maleic ester units. A disadvantage of such products is that reaction of the polyisocyanate may be followed by the formation of hydantoin, as a result of which the oligoester chains are cleaved. This in turn leads to a loss of mechanical properties on the part of the coating.
EP-A 0 849 301 describes aqueous binder mixtures based on polyaspartic esters and aqueous polyisocyanates. A disadvantage, however, is that these polyaspartic esters are not water-dispersible or water-soluble, and so cannot be provided in the form of storage-stable aqueous formulations. The coatings must therefore be produced by adding the aqueous polyisocyanate to the hydrophobic polyaspartic ester, then mixing the system with water and applying it immediately.
For reasons of improved processing, however, it would be desirable to be able to use water-soluble or water-dispersible aspartates or aqueous formulations thereof in such systems.